Heretofore, various methods have been utilized for applying indexing labels or tabs to various substrates. The most successful design incorporates a face stock laminated to a clear plastic film with a pressure sensitive adhesive for securement.
A convenient method of application involves the folding of the label over the edge of the folder or other substrate, the pressure sensitive adhesive providing securement on both sides. There are however several drawbacks to this method, one being that the label may be applied unevenly if the fold line is not parallel to the center line of the label. Even if the label is applied correctly, there is often a certain degree of wrinkling along the fold line because the paper face stock is not sufficiently stretchable to accommodate the bend. When this occurs there is a high probability of void spaces existing between the label and the substrate, allowing oxygen to enter and eventually degrade adhesion.
One solution has been to slit or score the label at a position where the fold is to be generated. This facilitates folding at the correct location as well as decreasing in some instances the amount of wrinkling by providing a complete separation of the face stock into two parts. Such a teaching is found in U.S. Pat. No. 3,805,426. What this method lacks however is the ability to adhere correctly to substrates of considerable thickness. When a single cut or score is made and the label is applied to a folder, card, etc. having a thickness somewhat greater than a sheet of paper, wrinkling will still occur because there is no mechanism for eliminating bending of the face stock at either corner of the edge surface. That is, if the slit is positioned at the center of the edge surface, the face stock is still required to bend at either corner. Naturally, the greater the thickness of the substrate, the more pronounced is the wrinkling and eventual degradation of adhesion.
Illustrative of cutting, perforating or otherwise introducing a fold line in the bending of a label is U.S. Pat. No. 3,348,324. This teaching also contains the limitations mentioned above. U.S. Pat. No. 3,324,823 discloses the use of "Mylar" (polyester film) in a label structure but otherwise does not deal directly with the problem of wrinkling of the label. U.S. Pat. Nos. 1,798,321 and 4,143,477 both make use of fold lines in a tab label. However there is no teaching or suggestion of how to eliminate wrinkling caused by the fold. U.S. Pat. No. 3,001,306 provides for index tabs comprising a laminate having extensions which are adhered to either side of a sheet material. There is no mention of a method for eliminating wrinkling.
From the above it is apparent that there is a need for a method of applying a label to a substrate having a considerable thickness, whereby wrinkling of the label is eliminated. This need is met by the instant invention.